Severe Weather

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Severe Weather

• Tornadoes

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Introduction

• Severe weather events can occur at any time and
  are difficult to predict.
• These include some of the most costly weather
  events
• Tornadoes
• Hurricanes
• Extra tropical cyclones
• Blizzards
• Thunderstorms

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Tornadoes are one of nature's most violent
storms.
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A tornado is a violently rotating column of air
extending from a thunderstorm to the ground.
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How to build a Tornado?

• To have a tornado you need a thunderstorm. To
  have a thunderstorm you need three things
• instability
• an uplifting mechanism
• moisture in the mid to lower levels of the
  atmosphere

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Instability

• An unstable air mass is warm and moist near the
  ground and relatively cold and dry in the upper
  atmosphere. If an air mass is unstable, air that
  is pushed upward will continue upward.
• Question? What role does humidity play in
  conditional instability?

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Lift

• Lift is the mechanism that pushes the air upward.
• cold fronts (drylines)
• mountains
• converging air (as in Florida)
• differential heating.
• The most common of these is differential heating.
  Differential heating takes place when portions of
  the earth's surface warm more readily than nearby
  areas. These "warm pockets" are less dense then
  the surrounding air and will then rise.

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Moisture

• The final ingredient is moisture. As air rises in
  a thunderstorm updraft, moisture condenses into
  small water drops, which form clouds. When the
  moisture condenses, heat is released into the
  air, making it warmer and less dense than its
  surroundings. This lets the updraft continue
  rising.

Instability !!
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Life of a Thunderstorm

• Three stages in a thunderstorm's life.
• The cumulus stage is the development stage.
• The mature stage is the height of the storm's
  strength.
• This is the stage in which tornadoes form.
• The dissipating stage is dominated by the
  downdraft. Rains itself out.

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The birth of a tornado

• Winds blowing from different directions at
  different altitudes and speeds, cause the air
  around it to begin rotating horizontally.
• When this horizontal column of air meets one of
  the strong updrafts associated with a
  thunderstorm, it can become twisted and bent
  upward. This mass of rotating air is known as the
  mesocyclone. It starts at ground level.

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A Tornado is born

• As the mesocyclone strengthens it extends further
  downwards. At the same time, it is becoming more
  compact which is causing it to spin faster and
  faster. If this process continues, then the
  mesocyclone will reach to the ground, spawning a
  tornado.

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Tornadoes

• Local storm of short duration
• Features
• Rotating column of air that extends down from a
  cumulonimbus cloud to the ground
• (Funnel cloud doesnt reach to the ground)
• Low pressures inside causes the air to rush in
• Winds approach 300 miles per hour
• Smaller suction vortices can form inside stronger
  tornadoes

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Multi-Vortex
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Tornado Occurrence and Development

• Average of 770 each year in the United States
• Most frequent from April through June
• Associated with thunderstorms
• Exact cause is not known
• Formation of tornadoes
• Occur most often along a cold front
• Associated with huge thunderstorms called
  supercells

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Tornadoes Characteristics

• Diameter between 150 and 600 meters (500 and 2000
  feet)
• Speed across landscape is about 45 kilometers (30
  miles) per hour
• Cut about a 10 km (6 miles) long path
• Maximum winds range beyond 500 kilometers (310
  miles) per hour
• Intensity measured by the Fujita intensity scale

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The most violent tornadoes are capable of
tremendous destruction with wind speeds of 250
mph or more.
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Tornado damage paths can be in excess of one mile
wide and 50 miles long.
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Tornado Watch Conditions are favorable for the
development of tornadoes within the areas and
times specified in the watch. Be prepared to take
shelter, preferably in the lower level of a
sturdy building.
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Tornado Warning One or more tornadoes are
occurring. If you are in the path of a tornado,
take emergency precautions immediately. If you
are near the area specified in the warning, be
alert for the development of additional tornadoes
or severe thunderstorms.
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Tornado Deaths per decade
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In an average year, 800 tornadoes are reported
across the United States, resulting in 80 deaths
and over 1,500 injuries.
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The United States has by far the most tornadoes
of any country in the world.
Over 90!
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Average Tornadoes per Year per 10,000 sq mi
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Tornadoes are most likely to occur between 3 and
9 p.m. but have been known to occur at all hours
of the day or night.
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Annual tornadoes per state for 45 yr period
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Per state per 10,000 sq mi
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Tornado Outbreak
Main Season April, May, June
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The Fujita Scale

• The Fujita Scale measures a tornado on a scale
  from 0 to 5 based upon the amount of damage
  inflicted upon man-made structures. You CANNOT
  look at a tornado and determine its intensity.
• Intensity Wind Speed Damage
• F0 40-72 mph Light
• F1 73-112 mph Moderate
• F2 113-157 mph Considerable
• F3 158-206 mph Severe
• F4 207-260 mph Devastating
• F5 261-318 mph Incredible
• It should be noted that there is a hypothetical
  F6 with winds from 318 mph to Mach 1. This is
  hypothetical. It has not been proven to exist and
  may never be proven to exist, however, there is
  enough evidence supporting the idea to mention it
  here.

Has been changed!
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Fujita Intensity Scale
Percentage of Tornadoes-blue
Percentage of Deaths -red
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Only 2 of all tornados are violent tornadoes,
but they cause 70 of all tornado deaths. Violent
tornados can live for more than 1 hour and have
winds greater than 205 mph.
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Paths of Tornadoes
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Super Tornado Outbreak April 3-4, 1974
The biggest and costliest tornado outbreak in
U.S. history with 148 twisters touching down in
13 states. Before it was over 16 hours later, 330
people were dead and 5,484 were injured in a
damage path covering more than 2,500 miles
causing 600 million in damage.
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The Deadliest Tornado Outbreak was The Tri-State
Tornado Outbreak of March 18, 1925 that killed
689 people in Missouri, Illinois and Indiana.
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Myths and Facts

• MYTH Areas near rivers, lakes, and mountains are
  safe from tornadoes.FACT No place is safe from
  tornadoes.MYTH The low pressure within a
  tornado cause buildings to "explode" as the
  tornado passes.FACT Violent winds and debris
  slamming into buildings cause most structural
  damage.MYTH Windows should be opened before a
  tornado.FACT This allows damaging winds to
  enter. Forget about the window and get to shelter
  immediately

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?
Questions?
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Fujita Tornado Intensity Scale
Category F0 Gale tornado (40-72 mph) light
damage. Some damage to chimneys break branches
off trees push over shallow-rooted trees damage
to sign boards. Category F1 Moderate tornado
(73-112 mph) moderate damage. The lower limit is
the beginning of hurricane wind speed peel
surface off roofs mobile homes pushed off
foundations or overturned moving autos pushed
off the roads. Category F2 Significant tornado
(113-157 mph) considerable damage. roofs torn
off frame houses mobile homes demolished
boxcars pushed over large trees snapped or
uprooted light-object missiles generated.
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Fujita Tornado Intensity Scale
Category F3 Severe tornado (158-206 mph) Severe
damage. Roofs and some walls torn off
well-constructed houses trains overturned most
trees in forest uprooted heavy cars lifted off
ground and thrown. Category F4 Devastating
tornado (207-260 mph) Devastating damage.
Well-constructed houses leveled structure with
weak foundation blown off some distance cars
thrown and large missiles generated. Category
F5 Incredible tornado (261-318 mph) Incredible
damage. Strong frame houses lifted off
foundations and carried considerable distance to
disintegrate automobile sized missiles fly
through the air in excess of 100 yards trees
debarked incredible phenomena will occur.